Embryo cells not like peas in a pod

IN A mammalian embryo, all cells are equal - or so biologists believed. But a series of studies suggest that the fate of individual cells might be determined much sooner after conception than previously thought.

In some non-mammals, such as fruit flies, there are different concentrations of certain molecules in different parts of the egg. When the egg cell divides, the "daughter" cells use this as a kind of grid reference to work out where in the egg cell they have come from and what they should become. This pattern is inflexible: split an insect egg by pinching it in the middle and you don't get twins; you get a front end and a back end of the insect.

Mammalian embryos appear to be much more flexible. If you take a mouse embryo at the two-cell stage and destroy one of the cells, you still get a complete mouse. This ...

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